Bloating galaxies confound astronomers

By Rachel Courtland (Image: I Damjanov et al/NASA/GDDS) Astronomers continue to puzzle over the recent discovery of a strange population of dense, compact galaxies that existed in the early universe but are nowhere to be seen today. They suspect the galaxies somehow puffed up into the bloated behemoths we see around us, but new research shortens the timescale during which this mysterious swelling could have happened. In April, astronomers reported finding extremely compact galaxies as far back as 10 billion years ago, or 3.7 billion years after the big bang. The galaxies contained the same number of stars as modern, blob-shaped galaxies known as ellipticals – but were two to three times smaller on average. Now, observations have turned up compact galaxies roughly a billion years later, when the universe was almost 5 billion years old. Some, dubbed ‘red nuggets’, are extremely compact – weighing as much as modern ellipticals, but measuring as little as a tenth their size. “There’s nothing like this in the nearby universe,” says astronomer Roberto Abraham of the University of Toronto in Canada. “These things are a complete, out-of-left-field surprise.” Astronomers do not see any of these galaxies in surveys of the present-day universe, so they reason something must have happened to cause them to get larger without forming more stars. “The ideal way is to figure out a way to puff them up, to make them bigger without changing their mass,” says Abraham. “But it’s hard to come up with a model that can do that.” That’s because the galaxies had to puff up very quickly. Abraham and colleagues led by Ivana Damjanov of the University of Toronto studied galaxies at different eras and found that the compact galaxies had disappeared from the universe only 1.6 billion years after their own observations of the red nuggets. Various explanations have been proposed for the galaxies’ bloating, but none accounts for all of the observations. One possibility is that the galaxies got larger by colliding and combining with other galaxies. Such mergers tend to produce newborn stars, since the collision triggers gas clouds within the galaxies to start condensing. But since both the modern elliptical galaxies and the old compact varieties seem to be devoid of young stars, astronomers suggested that the mergers must have been ‘dry’ – occurring between galaxies with a dearth of star-forming clouds. Still, such mergers would increase the number of stars – and therefore the mass – in the resulting galaxies – and observations show compact galaxies and ellipticals are roughly equivalent in mass. What’s more, statistics dictate that some compact galaxies should survive to the present day without merging – but none are seen. “There is no known process that can make that big an increase in size” in so short a time, notes Alvio Renzini of Italy’s INAF-Astronomical Observatory of Padova, who was not associated with the study. “Dry merging helps a little but not enough.” Another possibility is that material in these galaxies gets redistributed as old stars lose mass by exploding as supernovae or sloughing off their outer layers. The gas ejected towards the edges of a galaxy could reduce the gravitational attraction to the galaxy’s centre, causing expansion. But even though the vast majority of stars in the galaxies are old, this ‘adiabatic expansion’ would take too long, says Damjanov. “The time you need in order to puff up galaxies by a factor of two or three is too long, and these galaxies don’t have the time,” she told New Scientist. She says a combination of mergers and expansion might solve the mystery,
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